In photographic silver salt diffusion transfer processes a donor sheet comprising a silver halide emulsion is exposed to a light image, then processed in close contact with an image receptor sheet with an interposed thin layer of a processing solution containing a developer and a silver halide solvent. In exposed areas of the donor emulsion, a silver negative image is developed, whilst in unexposed areas the undeveloped silver halide is dissolved to form a soluble silver complex which diffuses to the receptor. The receptor sheet contains catalytic particles, or nuclei, which catalyse the reduction of the dissolved silver complex by unused developer to form a positive silver image on the receptor.
The deposition of silver, or in some cases other metals, onto catalytic nuclei by reduction of a dissolved metal salt is generally called physical development. Photographic silver salt diffusion transfer processes use physical development to provide a positive image by one-step processing, and the image so obtained is usually required to be of neutral colour, rapidly formed, and of high density. These properties are controlled to a large extent by the size and nature of the physical development nuclei incorporated in the receptor sheet.
Physical development nuclei usually consist of silver, gold or other heavy metal, or their sulfides or selenides. The easilyprepared colloidal suspensions of these substances are commonly used, giving rapid development and image silver of high covering power, but suffer from the disadvantage of producing brown-coloured images. Various organic image toning agents have been suggested to overcome this problem, as also has the use of a fine particulate inert support for the nuclei, such as silica. Both of these methods may have disadvantages; for example organic toners may have undesirable photographic side-effects, and particulate supports may lead to unwanted opacity in the receptor.
While receptor nuclei of a larger particle size give a more neutral-toned image, the controlled precipitation of such nuclei from simple solutions is difficult to achieve reproducibly, and if the particle size exceeds an optimum value, the resulting nuclei have poor catalytic properties and produce image silver of low covering power on physical development.
It is, therefore, an object of the invention to provide an improved way of preparing these nuclei.